1. Field of the Invention
The present invention generally relates to a zoom lens system, and more particularly to a stepwise variable zoom lens system which has a short overall length and is suitable for compact electronic equipments.
2. The Related Art
In recent years, compact electronic equipments with photographic function, such as simple cameras, mobile phones and personal digital assistants (PDA), have been more and more popular. These compact electronic equipments have become so small that they require zoom lens systems to be configured therein be as short as possible in overall length and as simple as possible in structure.
U.S. Pat. No. 6,233,099 issued on May 15, 2001 discloses a zoom lens system which has a first lens group having a negative refractive power, a second lens group having a positive refractive power, a third lens group having a negative refractive power, and a fourth lens group having a positive refractive power in order from an object side. During zooming from the wide-angle zoom position to the telephoto zoom position, all the lens groups move along an optical axis in such a manner that the separation between the first lens group and second lens group decreases, the separation between the second lens group and the third lens group increases and the separation between the third lens group and the fourth lens group decreases. Because all the lens groups are made moveable for zooming, the lens system needs a complicate zoom control mechanism, which is not suitable for compact electronic equipments. Furthermore, all the moveable design of the lens groups is detrimental to shorten the overall length and simplify the structure of the lens system. More seriously, the first moveable lens group makes the lens system be easily damaged.